Helmet structures and methods

ABSTRACT

A personal protective item, such as a helmet, includes a substrate formed of a hexagonal structure comprising a plurality of hexagonal tubes, each hexagon tube having a first end and a second end and being formed by a plurality of walls, the first and second ends being defined by edges of the plurality of walls. A cylindrical end cap is provided on at least some of the edges of the plurality of walls. The personal protective item may further comprise supporting columns along the hexagonal tubes where the walls of adjacent hexagonal tubes meet. A pad for providing user comfort may also be provide., the pad comprising a plate that is deflectable relative to a substrate of the personal protective time, the pad being coupled to the substrate by one or more constant force springs. Parts of the protective item may be joined using splines and grooves.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Pat. ApplicationSerial No. 63/251,164 filed Oct. 1, 2021 and U.S. Provisional Pat.Application Serial No. 63/324,972 filed Mar. 29, 2022, the discloses ofwhich are incorporated herein as if explicitly set forth.

FIELD OF THE INVENTION

This application relates to structures and methods for use in protectiveequipment, including but not limited to helmets for use in recreationalactivities.

BACKGROUND

Designers of protective equipment are often faced with many conflictingrequirements and challenges. For example, helmets for sporting use areexpected to be lightweight, able to withstand and absorb significantimpacts of different types and directions, capable of providing air flowto the wearer’s head in use, as well as a comfortable and conformablefit that can accommodate variations in the size or shape of a user’shead within each specific helmet size. Considerations of style, and thelimitations of existing molding techniques in Expanded Polystyrene (EPS)molding, are also relevant.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

To easily identify the discussion of any particular element or act, themost significant digit or digits in a reference number refer to thefigure number in which that element is first introduced.

FIG. 1 is a view of an underside of a helmet shell according to someexamples.

FIG. 2 is an external view of part of the helmet shell of FIG. 1 in anarea above a user’s forehead.

FIG. 3 is a perspective view of an internal honeycomb structure of thehelmet shell of FIG. 1 , according to some examples.

FIG. 4 shows the orientation of a finished component of the helmet shellof FIG. 1 relative to a print bed, in some examples.

FIGS. 5A through 5D show four different cross-sections of a component ofthe helmet shell of FIG. 1 , as the cross sections being printed advanceduring the additive manufacturing.

FIG. 6 shows the reinforcement of a honeycomb structure, in which theintersections between adjacent walls have been reinforced, according tosome examples.

FIG. 7 shows the reinforcement of part of a helmet shell in which theintersecti ons between adjacent walls have been reinforced, in someexamples.

FIG. 8 is an exploded view of a constant-force pad to be used inprotective equipment, such as helmets, according to some examples.

FIG. 9 is a chart 900 showing the relationship between pad displacement(movement due to pressure from the wearer) and force applied, in someexamples.

FIG. 10A and FIG. 10B illustrate front and side views respectively ofthe pad of FIG. 8 , in an assembled configuration and in 3D printorientation.

FIG. 11 shows a perspective view of a helmet substrate 1100 according tosome examples.

FIG. 12 shows a perspective views of the helmet substrate of FIG. 11 .

FIG. 13 is a perspective view of a pad 1300 in which springs used toprovide the constant force deflection reside in the helmet substrate,according to some examples.

FIG. 14 shows a perspective view of a helmet substrate 1400 in whichhidden detail of post holes is shown, according to some examples.

FIG. 15 shows two polymer constant-force accordion-shaped springsdesigned for FDM fabrication.

FIG. 16 shows a perspective view of two parts of a helmet that are to bejoined together during assembly of the helmet, according to someexamples.

FIG. 17 shows an end view and two perspective views of an extrinsicspline, according to some examples.

DETAILED DESCRIPTION

Honeycomb (hexagonal) sandwich panels are commonly used for lightweightmechanical structures. In such panels, hexagonal interior walls areenclosed by parallel sheets to form a plate-like assembly. Honeycombsandwich panels are used where a high ratio of strength to mass isneeded, and are typically available in paper, aluminum, fiberglass andadvanced composite materials.

The desirable properties of sandwich panels extend to products producedby additive manufacturing. For example, total weight and specificstrength are key concerns for the manufacture of custom 3D-printed sportsafety helmets. For some products, holes through the structure arenecessary, such as in a bicycle helmet where user comfort depends onadequate airflow. Unfortunately, any holes in a sandwich panel greatlyreduces its mechanical properties by allowing crumpling in-plane.

For an additive manufactured helmet there are other problems withremoving all or part of the outer panels: knife-like walls orientedperpendicular to the inside that may pose a safety risk, difficulty insome additive manufacturing processes to print walls consistently whenthere is no peripheral support, and a tendency to propagate cracks whereinner and outer walls meet at ninety degrees. Simply making holes in theouter surfaces that are smaller than the hexagons defined by the wallsof a honeycomb structure retain many of these mechanical problems.

In some examples, provided is a personal protective item such as ahelmet. The personal protective item includes a hexagonal structurecomprising a plurality of hexagonal tubes, each hexagon tube having afirst end and a second end and being formed by a plurality of walls, thefirst and second ends being defined by edges of the plurality of walls,and a cylindrical end cap on at least some of the edges of the pluralityof walls. At least some of the hexagonal tubes may be open adjacent tothe end caps for ventilation. The end caps may be circular cylinders orcircular tubes, or may be elliptical in shape.

The personal protective item may further include an end wall coupled toat least some of the edges of the walls to close the ends of at leastsome of the hexagonal tubes, the edges of the walls of the hexagonaltubes adjacent to the end wall not having cylindrical end caps.

In some examples, the personal protective item may also includesupporting columns along the hexagonal tubes where the walls of adjacenthexagonal tubes meet. The supporting columns may taper from the firstend to the second end. A side of at least one supporting column may beflat to enhance 3D printability. A diameter of the supporting columnsmay also vary across the personal protective item.

The personal protective item may be assembled from at least a first partand a second part, the first part having a spline defined thereon andthe second part having a groove defined therein for receiving thespline.

The personal protective item may be assembled from at least a first partand a second part and an extrinsic spline, the first part having groovedefined therein for receiving the spline and the second part having agroove defined therein for receiving the spline.

In some examples, the personal protective item includes a pad forproviding user comfort, the pad including a plate that is deflectablerelative to a substrate of the personal protective time, the pad beingcoupled to the substrate by one or more constant force springs. The padmay include a number of posts mounted to the plate, the posts beingreceived by post holes defined in the substrate.

In some examples, a personal protective item such as a helmet includesan inner wall having a first aperture defined therein, an outer wallhaving a second aperture defined therein, a honeycomb structure locatedbetween the inner wall and the outer wall, the honeycomb structureincluding a plurality of hexagonal tubes having first ends and secondends, the first and second ends of a first group of the hexagonal tubesbeing exposed through the first aperture and the second aperture, andend caps located on the first and second ends of the first group ofhexagonal tubes. The first and second ends of a second group ofhexagonal tubes are coupled to the inner wall and the outer wallrespectively without having end caps.

The hexagonal tubes may comprise tube walls, and the personal protectiveitem may further include supporting columns along the hexagonal tubeswhere the tube walls of adjacent hexagonal tubes meet. The supportingcolumns may taper along the length of the hexagonal tubes, and adiameter of the supporting columns may vary across the personalprotective item.

The personal protective item may be assembled from at least a first partand a second part, the first part having a spline defined thereon andthe second part having a groove defined therein for receiving thespline.

The personal may further include a pad for providing user comfort, thepad includes a plate that is deflectable relative to a substrate of thepersonal protective time, the pad being coupled to the substrate by oneor more constant force springs.

Other technical features may be readily apparent to one skilled in theart from the following figures, descriptions, and claims.

FIG. 1 is a view of an underside of a helmet shell 100 according to someexamples. The helmet shell 100 has been additively manufactured toinclude exterior walls (not shown in FIG. 1 ) and interior walls 102,between which are provided a honeycomb structure as will be described inmore detail below.

Ventilation holes 106 are provided in the walls 102. The holes 106correspond to the walls of the honeycomb structure, allowing air flow tothe user’s head from the outside. The holes 106 thus form a honeycombpattern corresponding to the interior honeycomb structure. The openingsof the holes at the interior and exterior walls are reinforced bycylindrical or tubular end caps 104 formed at the upper and lower endsof the hexagons formed by the interior honeycomb structure. Thetransition from the walls of the honeycomb structure to the end caps 104may be filleted to discourage crack propagation between the honeycombstructure and the end caps 104.

The reinforcement to the otherwise exposed hexagonal edges of theinterior honeycomb structure provided by the end caps 104 improvesprintability and structural integrity without greatly hindering airflowfrom the exterior of the helmet shell to the user’s head. The end caps104 may have a circular, elliptical or some other cross-sectional shape.

In some examples, Fused Deposition Modelling (FDM) is used to make thehelmet shell 100. Circular or elliptical shapes are advantageous to FDMbecause imprecision of starting and stopping filament precisely(retraction) does not weaken the end cap. Circular or elliptical endcaps are tougher than planar endcaps and present rounded edges to theexterior surface instead of sharp edges. In some examples, the helmetshell 100 comprises a number of pieces that are made separately usingadditive manufacturing, which are then assembled into the final helmetshell 100, for example as discussed below with reference to FIG. 16 andFIG. 17 . Additional components such as pads, straps and so forth arethen added to the helmet shell to make up the final helmet.

FIG. 2 is an external view of part of the helmet shell 100 in an areaabove a user’s forehead. The exterior of the helmet shell 100 includesan exterior wall 202, which has an aperture defined therein. Theunderlying honeycomb structure is exposed by the aperture defined in theexterior wall 202, with end caps 104 providing reinforcement of thehoneycomb structure. The holes 206 defined by the end caps 104 arecoupled to the holes 106 shown in FIG. 1 by the hexagonal tubes formingthe honeycomb structure.

As can be seen, the aperture defined by an edge 204 of the exterior wall202 does not follow the end caps 104 as for the interior wall 102. Thisis typically done for aesthetic or other design purposes. The end caps104 may, but typically do not continue underneath the exterior wall 202,since reinforcement of the interior honeycomb structure in that area isthen provided by the exterior wall 202.

Also shown in FIG. 2 is the plane 210 of a print bed, and a direction208 in which 3D printing proceeds as discussed below with reference toFIG. 4 .

FIG. 3 is a perspective view of an internal honeycomb structure 300 ofthe helmet shell 100, according to some examples. FIG. 3 shows the partof the helmet shell 100 shown in FIG. 2 from another direction and withthe exterior wall 202 removed. The normally hidden honeycomb walls 302are visible, with end caps 104 provided on both top and bottom edges ofthe honeycomb walls 302 in areas that have ventilation through thehelmet. The honeycomb walls 302 define a number of hexagonal tubesthrough which ventilation can be facilitated by the use of end caps 104

FIG. 4 shows the orientation of a finished component 402 of the helmetshell 100 relative to a print bed 404 in some examples. As can be seen,the additive manufacturing of the component 402 is arranged so that twoof the six honeycomb walls are, as far as possible, vertical to theprint bed. This orientation minimizes the worst-case overhang of thehoneycomb, in which walls of the honeycomb are horizontal to the printbed 404. This orientation is also illustrated in FIG. 1 by the printdirection 208 and plane 210 of the print bed.

FIGS. 5A through 5D show four different cross-sections of a component ofthe helmet shell 100, such as the front section shown in FIG. 2 and FIG.3 , as the cross sections being printed advance during the additivemanufacturing in direction 208. The cross sections shown are parallel tothe plane 210 of the print bed and advance in the direction 208.

FIG. 5A is a cross section that cuts across honeycomb walls 302 that arevertical in FIG. 2 . In FIG. 5B, continuing upwards, the verticalhoneycomb walls 302 have each diverged into two angled honeycomb walls504 on the interior of the helmet shell 100 but not the exterior. Thisis because the honeycomb orientation is somewhat angled relative to theplane 210 of the print bed. The end caps 502 on the interior of thehelmet shell 100 are now elliptical in cross section, they are on theend of a honeycomb wall that is angled to the plane 210 of the print bedand are thus also angled, and the cross section of an angled circularcylinder or tube is an ellipse.

In FIG. 5C continuing upwards in direction 208, the end caps 502 on theinterior surface are merging as two angled honeycomb walls 302 cometogether, until they have merged and transitioned to vertical end caps502 on a vertical honeycomb walls 302 as shown in FIG. 5D.

FIG. 6 shows the reinforcement of a honeycomb structure 600, in whichthe intersections between adjacent walls 602 have been reinforced. InFIG. 6 , exterior and interior end caps 104, 502 have been omitted forpurposes of clarity. As shown, the intersections between adjacent walls602 have been reinforced by cylindrical, conical or frustoconicalcolumns 604. Cylindrical or conical reinforcement may also be used whereinterior walls meet the exterior.

Providing columns 604 at the intersections of walls 602 in a honeycombstructure 600 can improve the specific energy absorption of a sandwichpanel including the honeycomb structure 600, or of the honeycombstructure 600 itself. Such integrated reinforcements are harder toproduce by traditional sandwich panel construction, but are well suitedto additive manufacturing. Such reinforcement is aesthetically andstructurally well suited for combination with end caps 104 parallel tothe head surface, and may be sized to be visually hidden by the end caps104.

In the case of frustoconical columns 604, the diameter of a columnadjacent to the inner surface of the helmet shell 100 and the diameterof the column 604 adjacent to the outer surface of the helmet shell 100can be varied from one column to another column around the helmet shell100, or for different sections of the helmet shell 100, to optimizedesired energy absorption and density. The end caps 104 may also be ofvariable diameter or not present everywhere. The diameters of end caps104 at an exterior surface of the helmet shell 100 may but need notmatch the diameter of end caps 104 at the interior surface of the helmetshell 100. As before, the transition between end caps 104 and columns604 may be filleted to reduce crack propagation.

In some examples of a bicycle helmet using a carbon-fiber reinforcedpolymer, the end caps 502 parallel to the scalp of a user and near theskin, and end caps 104 at the exterior surface of the helmet shell 100,have outer diameters of 3 mm and 4 mm respectively. This geometry isinterpolated to form columns 604 perpendicular to the head where threeor more walls come together. The surfaces may be constructed to smoothlyvary between the diameters.

FIG. 7 shows the reinforcement of part of a helmet shell 700, in whichthe intersections between adjacent walls 702 have been reinforced, insome examples. FIG. 7 shows a part of the helmet shell 100 near thehelmet exterior as seen from inside, with internal material removed toshow detail. The left side of FIG. 7 is ventilated, with holes 206, andthe right side is unventilated, with the exterior wall 202 covering theinternal honeycomb structure. End caps 104 are present in the ventilatedarea, but not where the exterior wall 202 is present. Columns 704 ofslightly smaller diameter than the end caps 104 can be observed whereverthree walls 702 of the hexagonal structure meet.

In an FDM printer, material must be extruded on top of already extrudedmaterial to have something to which to attach. Having no material belowthe nozzle (called “overhang”) leads to gross defects with materialbeing placed away from where it is intended. Some of the posts 708 havea flattened side 706 to enable FDM printing with less overhang. Acylinder having a nearly horizontal axis can create such a situation,where the bottom of the cylinder is too much “printing on air”. As shownin the image, the posts 708 have been adjusted to be flatter in waysthat reduce overhangs. Such adjustments permit easier manufacture of thedevice using additive manufacturing. The alternative is to print extramaterial that has to be manually removed later, which adds cost andcomplexity.

While the flattened sides 706, which are nearly horizontal during 3Dprinting, is at risk for print defects, this is less so than a roundcolumn would be. That is because, for the flattened side 706, thefilament is extruded along a shorter path between stable endpoints,whereas the extrusion for a cylinder has to follow a relatively longelliptical cross-section far away from support.

In FIG. 7 for example, in which the print bed is located below thefigure, it can be seen that the sides of the posts 708 that haveflattened sides 706 are the columns that have angled walls 702intersecting with the posts 708 from below. The columns 704 that do nothave flattened sides have vertical walls 710 below the column 704, whichprovides support for material deposition.

FIG. 8 is an exploded view of a constant-force pad 800 to be used inprotective equipment, such as helmets, according to some examples. Padsare used in many applications to isolate a rigid surface from a humanbody part. For example, foam pads are commonly incorporated into sportsgear, such as helmets, to provide user comfort. Pads fill the gapbetween the substrate and the body to allow equipment to be worn despitehaving an imprecise fit. Pads are commonly made of a foam. There may beadditional layers to control the force-displacement curve, wick sweat,adhere to the substrate, provide an attractive appearance, and so on.

Pads are most comfortable when they provide a consistent and controlledpressure. When sports equipment has a poor fit, the pressure may be toosmall (or zero) such that the equipment is not held in place correctly.Alternately, pressure that is too high is uncomfortable or painful. Foampads generally transmit more force as they get squeezed into a smallervolume. However, this limits the comfortable range of motion. While aconventional pad allows choosing a density to increase or decreasepressure, this cannot guarantee comfort as the ideal density will stilldiffer with fit, and pressure may not be evenly distributed across thepad.

One possible approach is to use a viscoelastic material (“memory foam”)so that pressure points even out over time. A potential problem withmemory foam is its reduced ability to sustain constant pressure overtime.

Instead of a foam, pressure may be spread using a relatively stifferpanel that distributes load over the entire pad surface. The stiffnessmay be chosen to allow some curvature (e.g., across the forehead), whilestill distributing force at small scales. Deflection of the pad isarranged to provide a flat force-displacement curve, as described inmore detail below with reference to FIG. 9 .

The pad 800 shown in FIG. 8 comprises a relatively stiff and smoothplate 802 with rounded edges suitable for prolonged contact with skin,which distributes load evenly to prevent pressure points, posts 804 forcoupling the pad 800 to the helmet, and a spring 806 that provides aflat force-displacement curve as described with reference to FIG. 9 .

Each post 804 includes a conical base 808 for stability, which mateswith a conical upper edge 1104 of a post holes 1102 in the helmetsubstrate as shown in FIG. 11 and FIG. 12 , a body 812 that slides intothe post holes 1102 in the substrate to permit the pad to deflect andretract toward the substrate under pressure, and a conical split snapfeature 810 that permits insertion into but not retraction of each post804, to hold the pad 800 in place on the helmet substrate.

The spring 806 comprises primary walls 814 that are aligned with theboundary of the pad 800 to permit movement of the plate 802 relative tothe helmet substrate, as well as a secondary wall 816 that preventsinversion of the primary walls 814 where the primary walls 814 meet thesubstrate. In one example the components of the pad can be 3D printedsimultaneously and bonded thermally by proximity.

To obtain a force-displacement curve that is flat instead of stiffeningwith displacement, the primary walls 814 comprise an angled first wall820 and an angled second wall 822 that meet at a waist 818, wherebuckling occurs when pressure is applied to the pad 800. In contrast toa traditional pad, the material properties only affect stiffness at thepoint (the waist 818) where buckling occurs. The buckling mechanismdescribed in this example is an inward bend in the pad primary walls814, but other arrangements are possible.

The material used for construction of the pad 800 can be quite stiffrelative to traditional foam, but foam, wicking fabric or aestheticmaterial may also be laser, knife or die-cut and adhered to the surfaceof the plate 802 opposite to the posts 804. A preferred material for thepad is polyether thermoplastic polyurethane (TPU) of shore 95A durometerwhich allows a smooth yet flexible surface and no additional materialnecessary.

Because the spring 806 has a flatter force-displacement curve than foam,the range of displacement can be larger than a traditional pad, withoutdiscomfort. The recess for the pad in the substrate and the depth of theholes for receiving the mounting posts can also be designed toaccommodate the entire retracted pad 800 all the way to the adjacentsurface of the substrate.

The secondary wall 816 prevents the primary walls 814 from turninginside-out (inversion) when the pad 800 is deeply depressed. Anothersolution would be to thicken the perimeter of the first wall 820 in thearea that comes in contact with the recess in the substrate. Suchthickening prevents undue expansion of the first wall 820 withoutaffecting the apparent stiffness, because the compliance of the primarywalls 814 is primarily at the waist 818.

FIG. 9 is a chart 900 showing the relationship between pad displacement(movement due to pressure from the wearer) and force applied, in someexamples. The chart 900 shows displacement of the pad on the x-axis inresponse to increasing force on the y-axis. The relationship betweenincreasing force and increasing displacement for the pad 800 and otherpads contemplated herein is shown by line 902. The conventional linearspring response is shown by the line 904. As can be seen, there is arange of acceptable or preferred forces for the pad 800 as shown in thechart 900 by the upper limit 906 and lower limit 908. When the force inresponse to displacement of the pad is too low, the pad fails tostabilize the personal equipment (such as a helmet) relative to thebody. When the force in response to displacement is too high, it isuncomfortable and chafes, for example by reducing blood flow.

An ordinary foam pad creates a linear relationship between displacementand force, as seen by the linear spring response line 904. The stiffnessof the foam/spring can be arranged to match the slope of the line toavailable or preferred displacements. However, regardless of thestiffness, there is a narrow range of displacements within theacceptable range of force, as can be seen. In comparison, a nearconstant-force spring is able to maintain a comparable force asdisplacement increases, allowing a larger range of displacements to beacceptable to the wearer than can be achieved by a linear response.

As can be seen from FIG. 9 , the constant force response illustrated byline 902 is not perfectly constant as displacement increases. Inparticular, there is a relatively steep initial increase in the forcerequired to create initial displacement, followed by some variation ofthe generally flat line 902 within the acceptable range of force, with arapid increase in the force required for additional displacement beyonda certain point, due to densification. “Constant force response” is thusto be understood in this context.

FIG. 10A and FIG. 10B illustrate front and side views respectively ofthe pad 800 of FIG. 8 , in an assembled configuration and in 3D printorientation. As can be seen, the posts 804 extend above the spring 806on the side of the pad 800 opposite to the plate 802. Also shown in FIG.10B is the split snap feature 810 illustrating the slot 1002 formed inthe posts 804 to permit the two sides of each post 804 to flex inward topermit insertion of the post 804 into the helmet substrate. The end ofthe post 804 is thus able to snap into place in a post hole while theundersurface of the conical split snap feature 810 prevents withdrawalby engaging a corresponding surface in the post hole.

FIG. 11 shows a perspective view of a helmet substrate 1100 according tosome examples. The substrate 1100 forms part of a helmet shell 100 forexample. The substrate 1100 includes three post holes 1102 sized andlocated to receive the posts 804 of the pad 800. The post holes 1102 aredeep enough to allow each post 804 to fully extend into its post hole1102 when the pad 800 is under pressure. Post hole upper edges 1104 areconical to facilitate post insertion and also for engagement with theconical post bases 808 when the pad is fully depressed, to provideadditional lateral support to the pad 800 when fully depressed. Theundersides of the split snap features 810 engage corresponding surfacesin the post holes 1102 to prevent withdrawal of the posts 804 from thepost holes 1102.

Also defined in the substrate is a recess 1106 that matches the overallshape of the pad 800. In some cases, the floor of the recess 1106 doesnot correspond to the entire shape of the pad 800 as shown, whichpermits the pad 800 to overlap with other functional equipment areas,such as the strap support 1108 seen in the figure.

FIG. 12 shows a perspective view of the helmet substrate 1100 of FIG. 11, in which hidden detail of the post holes 1102 is shown. A lower region1204 of each post hole 1102 is sufficiently wide so as to fullyaccommodate the split snap feature 810. An upper surface 1202 of thelower region engages the undersurface of the conical split snap feature810 to prevent post withdrawal, while allowing each post 804 to slide into its full depth.

The apparatus shown in FIG. 8 and FIG. 10A & B to FIG. 12 combines thepad surface, constant force spring, and the conical retention snaps intoa single part. These functions may instead be separated to allowindependent fabrication, different materials, alternative assemblymethods, end user adjustments, or greater depth of travel. FIG. 13 toFIG. 15 together illustrate an alternate constant force padimplementation in which springs are separate parts that reside in thehelmet substrate for increased range of motion.

FIG. 13 is a perspective view of a pad 1300 in which the springs used toprovide the constant force deflection reside in the helmet substrate,according to some examples. The pad 1300 combines the functions of theplate 802 and posts 804 of the pad 800 of FIG. 8 , excluding the spring806. As before, the pad 1300 includes a relatively stiff and smoothplate 1302 with rounded edges suitable for prolonged contact with skin,and for distributing load evenly to prevent pressure points. The pad1300 includes posts 1304 with partial conical bases 1306 for stability,which mate with a conical upper edge 1406 of post holes 1402 (see FIG.14 ) in the helmet substrate. In some examples, the posts 1304 shown inFIG. 13 slide into post holes 1402 in the helmet substrate againstsprings 1502 (see FIG. 15 ) in the post holes 1402, to permit the pad1300 to retract toward the substrate under pressure. The posts 1304 inFIG. 13 can however also be used with an outer spring arrangementattached to the pad 1300 as describe above with reference to FIG. 8 .

The posts 1304 include four-way conical split snap features 1308 thatpermit insertion of a post 1304 into, but not retraction of, a post 1304from a post hole 1402, to hold the pad 1300 in place. Arches 1310 aredefined in the post walls and holes 1312 are provided in the plate 1302,for example under the posts 1304, to reduce overall weight. An uppersurface 1314 of the split snap feature 1308 may have an angled outerportion to permit insertion of the posts 1304 into the post holes 1402,and a flat upper portion to engage a spring 1502.

As before, the surface of the plate 1302 on the other side from theposts 1304 may have a material bonded thereto to enhance comfort, forexample in the case of prolonged skin contact.

FIG. 14 shows a perspective view of a helmet substrate 1400 in whichhidden detail of post holes 1402 is shown, according to some examples.FIG. 14 shows the post holes 1402 as if seen from within the helmetsubstrate. The post holes 1402in FIG. 14 are functionally equivalent tothe post holes 1102 in FIG. 11 , and are appropriately sized (forexample with a larger diameter and deeper depth) to accommodate a spring1502 in each post hole 1402. Each post hole 1402 includes a chamfer 1408at the bottom that encourages a spring 1502 to remain centrally aligned.

The post holes 1402 are deep enough to allow each post 1304 to fullyextend into its post hole 1402 on top of a compressed spring 1502 whenthe pad 1300 is under pressure. Post hole upper edges 1406 are conicalto facilitate post insertion of a the posts 1304 and also for engagementwith the post conical bases 1306 when the pad 1300 is fully depressed,to provide additional lateral support to the pad 1300 when fullydepressed. The undersides of the split snap features 1308 engagecorresponding surfaces in the post holes 1402 to prevent withdrawal ofthe posts 1304 from the post holes 1402.

A lower region 1404 of each post hole 1402 is sufficiently wide so as tofully accommodate the split snap feature 1308 and a spring 1502, whichmay expand radially when compressed.

Any type of spring may be used within the post holes 1402, butpreferably constant force springs are used.

FIG. 15 shows two polymer constant-force accordion-shaped springs 1502designed for FDM fabrication. Each spring 1502 has a flat end 1504 thatis contacted by the end of the post 1304 and a tapered end 1506 forcontacting the chamfer 1408 at the bottom of the post hole 1402. Thesprings 1502 are hollow to permit the alternating spring walls (such asspring wall 1508 and spring wall 1510) to compress and expand towardsand away from each other during use. The flat end 1504 of each spring1502 also includes a hole 1514 to reduce weight and to allow users tomanipulate the springs 1502 during insertion or replacement.

The spring force provided by the springs 1502 is primarily as a resultof buckling occurring at the outer edges 1516 and waists 1518 betweenadjacent spring walls, such as spring wall 1508, 1510 and 1512. Thesprings 1502 may be any shape, but the octagonal radial accordion designillustrated in FIG. 15 reduces engagement with any imperfections in oron the walls of the lower region 1404 of the post hole 1402.

The spring 806 in FIG. 8 and the springs 1502 in FIG. 15 may be seen asa stack of effectively conical shells (also known as coned-disc springs,conical spring washers, or Belleville springs.) These compact springsfeature a force plateau near zero at the point where the spring “turnsinside out,” so the force-displacement profile can be tuned by modifyingthe dimensions of the spring for the desired displacement.

Any mechanism capable of plateauing force may be substituted, includingbut not limited to: cantilever springs, volute springs, leaf springs,torsion springs, wave springs, gas springs, main springs, or dilatant orauxetic foams and lattices.

The helmet shell 100 of FIG. 1 may be assembled from two or moreseparate parts. In this regard, FIG. 16 shows a perspective view of twoparts of a helmet that are to be joined together during assembly of thehelmet, according to some examples.

Additive manufacturing processes are typically limited to a “buildvolume,” for example, by the range of motion of the printer’s mechanism.There are also process-specific limitations to parts that includerequiring the part to have a flat surface to make contact with a “printbed,” disallowing certain angles, limitations on the number ofconcurrent materials used in a part, and disallowing enclosed regionsthat prevent raw material from being removed.

It is common to work around these limitations by splitting parts intomultiple pieces that undergo later assembly. These pieces can becombined in many ways, such as sewing, screwing, welding, solventwelding, snapping, etc. It is desired to have a robust technique thatrequires little skill, time and equipment for assembly.

As disclosed herein, parts may be joined with “dovetail” joints, or mayrequire external parts, for example “biscuit” or “butterfly” joints. Theword “spline” is used herein to describe any such mating part as used inspecific 3d-printed processes.

Assembling rigid pieces into a composite structure requires that theyslide relative to one another along some path with a cross-sectioninvariant to rotation and translation along the path. In woodworking forpractical reasons this is typically a straight line. The straight lineis a special case of a circular arc, which is a special case of a mostgeneral helical path. As helical curves are the most general family ofshapes, the word “helical” here may be understood to apply to straightor circular paths without loss of generality.

Splines may be partially printed directly into one part (intrinsic), orcreated as a separate component that holds larger pieces together(extrinsic). In the latter case, it can be useful to print the splineusing the same production process. Each part may incorporate any numberand type of splines for joining to neighboring parts.

Additive manufacturing makes joinery shapes possible that would not bepossible in traditional processes such as lathing or injection molding.The ideal path may be optimized by software in a manner appropriate tothe domain, rather than set rigidly in a Computer Aided Design (CAD)tool. For example, a 3d-printed helmet may conform to users’ heads forbest possible comfort and safety. Segments of the unique helmet may becombined using helical joints defined by software while still respectingprinting process constraints and mechanical performance requirements.The helical path may be placed optimally based on the contour of eachcustomer’s head.

The construction of an optimal helical path is domain specific. However,a practical special case is constructing a circular path that follows aplanar print bed in order to mate two parts formed from a referencedesign by an arbitrary coordinate space transformation. Three pointsdefine a unique circle, so when tolerances permit, a circular path canbe constructed by observing the transform at three predefined points,from which a circular and planar path can be recovered suitable forjoining. The strength of this technique is that the coordinate transformneed not be linear and is bounded only by the need to keep the printbeds planar.

Fused Deposition Modelling of a 3D-printed helmet is used as an example.The helmet is divided into individually printed sections, which areassembled by sliding together intrinsic splines or inserting extrinsicsplines. Splines may be constructed by any convenient process, notnecessarily 3d-printing. Parts may be solid, filled with a pattern, orhave thin walls. The material may be different for the spline and theconnecting parts. Once installed splines may be held in place in manyways including being welded, solvent welded, glued, shimmed, or frictionfit.

In this regard, FIG. 16 shows a first part 1602 and a second part 1604forming part of a helmet shell 1600. The first part 1602 has a helicalgroove 1606 defined therein into which an intrinsic spline 1608 onsecond part 1604 can be slid to mate the first part 1602 to the secondpart 1604. In the example first part 1602 is printed on a print bed thatis parallel to the plane dividing the two parts. The geometry of thegroove 1606 is thus constrained to the plane of the print bed of thefirst part 1602. The groove 1606 has an angled roof relative to theprint bed for the first part 1602 for printability, to reduce overhang.

The second part 1604 is printed on a print bed facing the reader. Secondpart 1604 also has a groove 1610 constrained by the print bed into whichan extrinsic spline (not shown) will be inserted for attaching anotherpart (also not shown).

FIG. 17 shows an end view and two perspective views of an extrinsicspline 1702, according to some examples. The end view of the spline1702, on the left, shows the print orientation of the spline 1702relative to a horizontal print bed. The spline 1702 is butterfly-shapedwith upper and lower faces that slope by at most 45 degrees to the printbed, for printability. The butterfly shape of the spline 1702 preventsseparation of the two parts when the left and right sides of the spline1702 are received in opposing grooves of the two parts that are to beassembled.

The perspective view in the middle shows a chamfer 1704 on the insertionend of the spline 1702 for easier insertion into corresponding groovesin the two parts that are to be assembled.

The perspective view on the right side shows a surface 1706 that isangled to match an exterior contour of the parts to be joined, toprovide a smooth finish and to conceal the hole formed by the grooves ofthe joined parts.

What is claimed is:
 1. A personal protective item, comprising: ahexagonal structure comprising a plurality of hexagonal tubes, eachhexagon tube having a first end and a second end and being formed by aplurality of walls, the first and second ends being defined by edges ofthe plurality of walls; and a cylindrical end cap on at least some ofthe edges of the plurality of walls.
 2. The personal protective item ofclaim 1, wherein the hexagonal tubes having cylindrical end caps areopen adjacent to the end caps for ventilation.
 3. The personalprotective item of claim 1, further comprising an end wall coupled to atleast some of the edges of the walls to close the ends of at least someof the hexagonal tubes, the edges of the walls of the hexagonal tubesadjacent to the end wall not having cylindrical end caps.
 4. Thepersonal protective item of claim 1, further comprising supportingcolumns along the hexagonal tubes where the walls of adjacent hexagonaltubes meet.
 5. The personal protective item of claim 4, wherein thesupporting columns taper from the first end to the second end.
 6. Thepersonal protective item of claim 4, wherein a diameter of thesupporting columns varies across the personal protective item.
 7. Thepersonal protective item of claim 4, wherein a wherein thecross-sectional shape of at least one column is modified forprintability.
 8. The personal protective item of claim 1, wherein theend caps and support columns have circular or elliptical cross-sections.9. The personal protective item of claim 8, wherein the end caps andsupport columns have cross-section shapes modified for improved 3dprintability by adding or removing faces.
 10. The personal protectiveitem of claim 1, wherein the personal protective item is assembled fromat least a first part and a second part, the first part having a splinedefined thereon and the second part having a groove defined therein forreceiving the spline.
 11. The personal protective item of claim 1,wherein the personal protective item is assembled from at least a firstpart and a second part and an extrinsic spline, the first part havinggroove defined therein for receiving the spline and the second parthaving a groove defined therein for receiving the spline.
 12. Thepersonal protective item of claim 1, further comprising a pad forproviding user comfort, the pad comprising a plate that is deflectablerelative to a substrate of the personal protective time, the pad beingcoupled to the substrate by one or more constant force springs.
 13. Thepersonal protective item of claim 12, wherein the pad comprises a numberof posts mounted to the plate, the posts being received by post holesdefined in the substrate.
 14. A personal protective item, comprising: aninner wall having a first aperture defined therein; an outer wall havinga second aperture defined therein; a honeycomb structure located betweenthe inner wall and the outer wall, the honeycomb structure comprising aplurality of hexagonal tubes having first ends and second ends, thefirst and second ends of a first group of the hexagonal tubes beingexposed through the first aperture and the second aperture; and end capslocated on the first and second ends of the first group of hexagonaltubes.
 15. The personal protective item of claim 14, wherein the firstand second ends of a second group of hexagonal tubes are coupled to theinner wall and the outer wall respectively without having end caps. 16.The personal protective item of claim 14, wherein the hexagonal tubescomprise tube walls, the personal protective item further comprising:supporting columns along the hexagonal tubes where the tube walls ofadjacent hexagonal tubes meet.
 17. The personal protective item of claim16, wherein the supporting columns taper along the length of thehexagonal tubes.
 18. The personal protective item of claim 16, wherein adiameter of the supporting columns varies across the personal protectiveitem.
 19. The personal protective item of claim 14, wherein the personalitem is assembled from at least a first part and a second part, thefirst part having a spline defined thereon and the second part having agroove defined therein for receiving the spline.
 20. The personalprotective item of claim 19 further comprising a pad for providing usercomfort, the pad comprising a plate that is deflectable relative to asubstrate of the personal protective time, the pad being coupled to thesubstrate by one or more constant force springs.